Among display devices, an electroluminescent device (EL device) is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials for forming a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
An organic electroluminescent device (OLED) has a structure comprising an anode, a cathode, and an organic layer between the anode and the cathode, wherein the organic layer may comprise a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. In the organic electroluminescent device, when a voltage is applied, holes and electrons are injected from the anode and the cathode, respectively, to the light-emitting layer, in which excitons having high energies are formed by a recombination of the holes and the electrons. The excition's energy puts the organic luminescent compound in an excited state, and light is emitted by energy released while returning from the excited state to a ground state. Accordingly, the most important factor determining luminous efficiency in an organic electroluminescent device is a light-emitting material.
The EL material can be categorized into a host material and a dopant material according to their functions. Generally, a device having the most excellent EL characteristic has a structure comprising a light-emitting layer formed by doping a dopant to a host. A host/dopant system is for increasing light-emitting efficiency by energy transfer from a host to a dopant. The host and dopant materials greatly influence the efficiency and lifespan of the EL device when using a dopant/host material system.
Until now, iridium(III) complexes have been widely known as a dopant of phosphorescent materials, and bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red, green, and blue light-emitting material, respectively.
However, considering EL characteristic requirements for a middle or large-sized panel of OLED, a development of a dopant compound of iridium complexes providing more excellent EL performance, such as long lifespan, high efficiency and high color purity, is still needed.
In particular, in order to realize full color display, to which an organic EL device is applied, a pixel configured to emit a particular color called “saturated” color is required. In particular, saturated red, green, and blue pixels are required. These saturated colors can be measured by using a CIE coordinate publicly known in the art.
In order to express more various colors, a color purity of each of red, green, and blue should be high. In case of red color, the closer to the red color coordinate near 0.680 (CIE X-coordinate standard), the higher the color purity is. Accordingly, a requirement for a suitable dopant compound is increased.
U.S. Pat. No. 7,951,472 discloses iridium complexes having 2-phenylquinoline-based ligand as a dopant compound of OLED, but iridium complexes having fused azabenzofluorene is not disclosed.